WASHINGTON — With its announcement Thursday that the decision to launch the Challenger was uncategorically ''flawed,'' the presidential commission ended the first phase of its investigation into the shuttle explosion that killed seven astronauts one month ago today.

This does not mean that the commission will not revisit the launch decision-making issue but only that for now it is about to move on and pursue other leads.

No public hearings are likely next week, said the commission chairman, former Secretary of State William Rogers, who declined to say what the next step of the investigation might be.

Presumably, the commission will direct its inquiries into the mechanical and technical nature of shuttle construction and flight.

The first order of business will be private meetings in which the members will attempt to sketch out an agenda, said commission spokesman Mark Weinberg. The commission will again break into subgroups, which will focus on specific issues and take field trips if necessary, Weinberg said.

Commission members Robert Hotz and Sally Ride may return to Kennedy Space Center sometime next week, Hotz said.

In the meantime, commission members who remain behind will hold ''working sessions'' in Washington.

To help in their work, the commission has hired 12 aides, including some professional investigators, said commission staff director Alton Keel.

Last week, some senators rebuked the commission not only for ''stonewalling'' Congress by holding private hearings but also for its staff shortage of ''gumshoes.''

Such criticism may be muted by the commission's action this week.

At the end of three days of public hearings Thursday, Rogers told top NASA officials that the commission members assume that NASA is ''working on improvements'' in the launch process.

More formal recommendations may come later. The commission has still to determine whether there was a cause-and-effect relationship between launch- process deficiencies and Challenger's explosion.

As the investigation unfolded, though, and those launch flaws became more glaring, commission members began to show signs of anger and impatience.

Apparently becoming more and more exasperated by testimony that showed a lack of communication between NASA and its general contractors, Rogers allowed that ''common sense'' did not seem to be playing much of a role in reaching important decisions.

At one point, the impeccably well-mannered Rogers seemed to be entirely out of character, breaking his self-restraint to render an unflattering judgment. This uncommon release of feeling was in reference to Robert Lund, an engineer-manager at Morton Thiokol Inc., where Challenger's suspect right booster was built. Lund initially recommended against a launch, then changed his mind after a meeting with superiors.